A child receives a polio vaccination near Jalalabad, Afghanistan, this week. Photograph: Parwiz/Reuters

A double vaccine could help combat polio in some of the world's most remote and conflict-torn regions, according to the World Health Organisation (WHO).

New research suggests giving a single vaccine shot to children who have already had the oral vaccine greatly boosted their immunity. The WHO said the combination strategy was being used in mass vaccination campaigns in some areas and for routine immunisations in developing countries.

"It could play a major role in completing the job of polio eradication once and for all," said Dr Hamid Jafari, WHO's director of polio operations, who led the study published on Thursday in the journal Science.

Oral polio vaccine has played an important role in the effort to eradicate the paralysing disease, as health workers have gone house-to-house, to refugee camps and to roadside checkpoints delivering the drops. The number of countries where polio regularly circulates dropped from 125 in 1988 to just three last year – Pakistan, Nigeria and Afghanistan.

But the threat is re-emerging in countries previously free of the highly contagious virus. The WHO in May declared an international public health emergency, citing outbreaks in at least 10 countries. Particularly of concern were Syria, Somalia and Iraq, where violence has complicated efforts to contain new cases.

The choice of vaccine has long been controversial. Wealthy nations have switched back to using only injected vaccine, which is made of "inactivated" or killed virus, for routine childhood immunisations because the oral vaccine contains weakened live virus that children can shed in their stools.

In developing countries where polio is still a threat, the oral version is cheaper and easier to use but a particular type of immunity, intestinal immunity, wanes so that children in high-exposure areas need repeated doses.

Jafari's team tested whether using both vaccines would protect better than one. The study involved nearly 1,000 children, from babies to 10-year-olds, in northern India in 2011, the last year that the country reported a case of polio. The children had previously received the oral vaccine. This time, they were randomly assigned to receive a dose of injected polio vaccine, another oral dose or no booster. Four weeks later, they all received what researchers called a "challenge" dose of oral vaccine to see how their bodies shed the weakened live virus.

The shots acted as a better booster for the children's intestinal immunity than giving them yet more vaccine drops – and those young people shed far less virus, key to cutting transmission in an outbreak, Jafari's team reported.

A similar study in 450 children in southern India last year reached the same conclusion, researchers reported in the Lancet last month.

Kenya put the strategy to its first real-world test in December. Health workers used both injected and oral vaccine as they sought to immunise 126,000 young children living in Somali refugee camps and nearby areas who were at risk from a polio outbreak spilling over the Somalia-Kenya border.

Similar campaigns are beginning in north-east Nigeria and should start soon in Pakistan, said Jafari and Dr Bruce Aylward, WHO's assistant director general for polio.

The injected vaccine is more expensive, Aylward said – about $1 (60p) to $1.90 a shot, under specially negotiated prices for low-income countries, compared to about 15 cents a dose for oral vaccine. But he said it was worth the investment if adding the shots eliminated polio in the last infected areas faster.